Source : Webster's Revised Unabridged Dictionary (1913)

Indefatigableness \In`de*fat"i*ga*ble*ness\, n.
   Indefatigable quality; unweariedness; persistency. --Parnell.

Source : WordNet®

indefatigableness
     n : tireless determination [syn: {indefatigability}, {tirelessness}]